1. Field of the Invention
The present invention relates generally to work holders, and particularly to clamping apparatus for releasably holding any one of a plurality of elongated articles having a range of sizes and shapes.
2. Description of the Prior Art
In order to function satisfactorily, a work clamp must be capable of withstanding the expected forces thereon and securely hold a workpiece in the installation in which it is to be used. In one popular version, a power operated pivotal jaw cooperates with a fixed base or jaw to releasably hold workpieces during handling thereof or performance of an operation thereon. It will be appreciated that in order to develop a sufficiently strong clamping force in this type of mechanism, it is desirable to employ a relatively long, jaw-carrying lever arm. This also is desirable for the reason that it facilitates complete and rapid withdrawal of the pivotal jaw for easy workpiece insertion and removal from the clamp.
Although this construction possesses the above advantages, it also has certain disadvantages. Particularly, a relatively long, jaw-carrying, pivotal lever arm requires considerable clearance beyond the end of the clamp structure to facilitate its movement. This, in turn, limits the number of such clamps that can be used in a given area and/or makes it difficult or impossible to employ this type of clamp with a work handling or operation performing device positioned close to the end of the clamp structure.
Pivotal jaw clamp constructions are desirable because their construction is relatively simple and inexpensive and yet are capable of developing high clamping forces. Thus, a clamp of this type capable of full jaw withdrawal and which requires a minimum of operational clearance for the movable jaw would be highly desirable.
Also known, are clamp constructions which employ a vertically disposed actuating cylinder located in relatively close proximity to the center line of the workpiece. In this instance, a holding clamp is fixed to an end of the rod operated by the actuating cylinder and lies in a plane perpendicular to the actuating rod. Longitudinal movement of the actuating rod changes the elevation of the plane of the holding clamp relative to the workpiece. Simultaneously, the actuating rod can be rotated about its longitudinal axis to move the clamp between an inactive position aligned with the longitudinal axis of the workpiece and an active position transverse of the workpiece. In the latter position, portions of the clamp are caused to overlie a flange of the workpiece and the cylinder is operated to draw the rod downwardly and pivot the clamp into engagement with the flange to grip the workpiece.
There are a number of drawbacks inherent in this design. In a first instance, the cylinder must be positioned closely adjacent the workpiece in order to minimize the cantilevered length of the clamp. Additionally, by reason of the cantilevered clamp design, the connection between the rod and the clamp is subjected to substantial bending stresses which are, in turn, transmitted to the cylinder. The stresses are harmful to seals within the cylinder and the life expectancy of the unit is thereby reduced. Furthermore, this design requires adequate room on either side of the cylinder extending in directions parallel to the workpiece in order to accommodate the rotary movement of the clamp in its horizontal plane. This substantially limits the placement and functioning of other machinery in the region of the workpiece, particularly, machinery which is necessary for operations on the workpiece itself.
The need presently exists to be able to releasably clamp any one of a plurality of elongated articles having a range of sizes and shapes. Stringers employed in the construction of aircraft wings represent a typical instance of such a need. In the course of manufacture, it is not unusual for successive stringers at a particular clamping location to be of a different size and shape than their predecessors. In the past, each newly sized and/or shaped stringer would require a new set-up to accommodate dimensional changes between the prior article and the subsequent article. This resulted in a time consuming and, therefore, costly procedure.
More recently, power operated pivotal jaw clamps have been devised which have enabled a plurality of clamps to be moved in a ganged manner between active and inactive positions. In this fashion, an article to be held can be selectively clamped or released without delay. Furthermore, the clamping apparatus can have sufficient built-in lost motion to accommodate the different sizes and shapes of the article being held.
Nonetheless, with the advent of ever more complex shapes into which stringers are formed, it has been necessary for the movable jaw of the clamp to move in a complex arc in order to avoid striking certain portions of the stringer while firmly clamping other portions thereof. By reason of the invention, articles being held having a particularly complex shape can be accommodated, and, the invention can be applied to a range of sizes and shapes of such articles.